1. Field of the Invention
The present invention relates to a process for spread spectrum coding and decoding a transmission, of the type in which a signal to be transmitted is phase modulated by a O-pi modulator controlled by a code formed of a sequence of binary values and the received signal is phase demodulated by means of a O-pi demodulator controlled by the same code. The code used at the receiver reception being synchronised with that used during transmission by correlating the receiver code with the received signal.
2. Summary of the Prior Art
In order to reduce the effect of jamming on transmitted signals and the risk of an intercepted signal being deciphered, it has been known to code the signal such that it covers a larger spectral spread. In such a process the spectral spread is realised by modulating, by means of a O-pi modulator, the signal by a code the spectrum of which is most important. On reception, the code used to control the demodulator is synchronised with the transmitter code by correlation with the received signal. That is delay means are used to progressively shift the code with reference to the received signal until a correlation peak is detected.
It is also clear that in order to be effective, the code must be long. Nevertheless, when the code used is lengthened the synchronisation operation on reception takes longer and quickly becomes prohibitive.